Optical fiber is becoming the transmission medium of choice for communication networks because of the speed and bandwidth advantages associated with optical transmission. Wavelength division multiplexing (WDM), which combines many optical channels at different wavelengths for simultaneous transmission as a composite optical signal in a single optical fiber, is being used to meet the increasing demands for more speed and bandwidth in optical transmission applications. With recent advances in optical networking technology, system manufacturers are now contemplating dense wavelength division multiplexing (DWDM) systems that include, for example, as many as 80 or more optical channels (i.e., wavelengths) in a single fiber. As such, DWDM optical transport technology is revolutionizing the telecommunication industry.
In view of the many advantages associated with the use of WDM and DWDM in long haul networks, WDM and DWDM technology is now being contemplated for use in short haul markets, such as metropolitan area networks and the like. Traditionally, short haul networks have been implemented as Synchronous Optical Network (SONET) rings using time division multiplexing (TDM) and the like. While such SONET rings perform well, the strong and consistent growth in demand for bandwidth and management of that bandwidth has outgrown the capabilities and capacity of SONET rings. As a result, there is a desire to extend, at the lowest cost possible, the enormous capacity and protocol independence of WDM and DWDM into these short haul ring networks.
In particular, there are many incentives for extending deployment of WDM and DWDM to the short haul networks in place of the existing time division multiplexed systems. For example, transport efficiencies can be achieved through deployment of packet or cell-based transport directly onto individual optical channels. Additionally, WDM and DWDM systems provide greater bandwidth and offer more flexibility in managing the dynamic bandwidth requirements of today's users.
However, implementation of WDM or DWDM in metropolitan area networks presents a unique set of challenges as compared to long haul network applications. For example, add/drop requirements are significantly greater in metropolitan area networks as compared to long haul networks because metropolitan area networks are typically more densely populated with users in a more geographically limited area. In addition, flow of traffic, diversity of traffic types, and dynamic changes in traffic levels associated with the adding and dropping of traffic further complicates the management of traffic in the metropolitan area network. Solving these problems using conventional WDM and DWDM techniques, such as those used in long haul applications, adds significant cost and complexity in the more cost-sensitive metropolitan area environment.
For example, conventional approaches to optical add/drop multiplexing are typically based on extracting the entire signal power for a selected wavelength at an add/drop node. Some examples of components used for optical add/drop multiplexing include in-line arrayed waveguide grating routers (AWG), fiber Bragg gratings (FBG), or Mach-Zehnder (MZF) filters, to name a few. However, these devices have several disadvantages that render them undesirable for practical applications in short haul applications, such as ring networks. For example, some of these disadvantages include: wavelength dependent loss; power penalties and other transmission impairments due to bandwidth narrowing and group velocity dispersion; limited spectral bandwidth, poor scalability, and high implementation costs.
In general, the economics of applying DWDM in inter-office (IOF) and access metropolitan applications depend very much on the cost of state-of-the-art optical components, such as optical multiplexers/demultiplexers, optical amplifiers, optical switches, and WDM sources. While the flexibility of bandwidth assignment is a key driver for this application, the cost of providing this capability using straightforward DWDM techniques appears prohibitively expensive for this more cost sensitive environment.
Consequently, a more cost competitive and technologically feasible solution for adding and dropping optical signals is required in order to realize the benefits of WDM or DWDM in metropolitan area networking.